It is known to separate distillate fractions from crude oils and by refining to free from components which impair colour, storage stability or aging stability or other properties essential to the purpose for which they are to be used. Known refining processes for separating off such undesirable components provide means for treating the distillate fractions with a selective solvent for aromatic hydrocarbons, such as furfurol, liquid SO.sub.2, phenol, N-methyl pyrrolidone, tetramethylene sulphone and the like; this treatment yields a purified solvate and an extract containing the undesirable components. In general, such solvates are further purified by treating them with concentrated sulphuric acid and adsorption agents, such as bleaching earth, after which they are usually termed raffinates. These refining processes can also be replaced, or supplemented, by hydrogating the distillate fractions or solvents usually in the presence of carrier catalyst under low to higher severity operation conditions. Furthermore, paraffin-rich distillate fractions are dewaxed prior to or after refining until a certain pourpoint has been reached, usually by dissolving the distillate or solvate in a solvent mixture, cooling this solution to at least the desired pourpoint, separating off the thereby deposited paraffin crystals and finally by separating and recovering the solvent mixture from the filtrate. With the aid of the refining process which has been generally described in the aforegoing, different mineral oil products, e.g. lubricating oils and transformer or electro-insulating oils, may be produced from distillate fractions of a suitable boiling range. The production of electro-insulating oils which are used for filling transducers, switches and transformers (herein after called transformer oils) according to the known refining processes, however, leads to difficulties since high demands are made of such transformer oils by the VDE specifications (Association of German Electrical Engineers) No. 0370/10.66 or DIN No. 51 507. The most important of the required values are as follows:
______________________________________ Density, gr/ml at 15.degree. C. max. 0.890 Kinematic viscosity, cst at 20.degree. C. max. 30 at -30.degree. C. max. 1800 Flashpoint (top dead centre according to Marcusson), .degree.C. min. 140 Corrosive sulphur 0 Neutralization No., mg KOH/g 0 Loss angle, tan max. . 10.sup.-3 Baader aging test (DIN 51 554) 28 hours at 95.degree. C. Saponification No., mg KOH/g max 0.60 Loss angle, tan max 200 . 10.sup.-3 Sludge content, % by wt. max 0.05 ______________________________________
The international valid specifications for transformer oils as well as a few problems concerning their production are explained in more detail in the journal "Electrotechnik und Maschinenbau" (Electrical Engineering and Mechanical Engineering), 1971, No. 7, pages 290 to 300. In view of the high requirements which are particularly made of the aging stability of transformer oils, the distillate fractions must be subjected to high-severity operation. In the course of this operation, natural antioxidants which are contained in many distillate fractions, pass with the undesirable components into the extract.